1. Field of the Invention
This invention relates to a combustion control apparatus for a spark ignition type 2-cycle internal combustion engine. Fresh air mixed with fuel mixed with fuel in a combustion chamber can be self-ignited at an ignition timing preferable for the operation of the internal combustion engine at least in a low load operation region to effect active thermal atmosphere combustion. When it is required to stop the internal combustion engine, the combustion control apparatus stops the internal combustion engine quickly. In addition, the combustion control apparatus provides increased starting performance for the internal combustion engine.
2. Description of Background Art
In a spark ignition type 2-cycle internal combustion engine, an exhaust port and a scavenging port opened or closed by a piston are formed on an inner peripheral face of a cylinder bore. Fresh air mixed with fuel prepressurized in a crank chamber is supplied from the scavenging port into a cylinder chamber while combustion gas in the cylinder chamber is exhausted from the exhaust port. The fresh air mixed with fuel compressed in the cylinder chamber is ignited by an ignition plug.
In such a conventional spark ignition type 2-cycle internal combustion engine, if the exhaust port is increased in size to set the output power and the efficiency in a high speed, high load operation region to values higher than high levels, then, in a low load operation region, the amount of unburned hydrocarbons in the exhaust gas is increased by blow-by of fresh air mixed with fuel or unstable combustion. In addition, the fuel consumption is increased.
In order to eliminate this, the present inventors have developed an internal combustion engine including an exhaust control valve which serves as exhaust path opening adjustment means. The exhaust control valve is driven to an exhaust path opening corresponding to an internal combustion engine speed and a throttle valve opening. Therefore, at least in a low load operation region, when an exhaust port is closed by a piston, the cylinder pressure is controlled appropriately to ignite fresh air mixed with fuel in the combustion chamber with the thermal energy of the combustion gas remaining in the combustion chamber. Therefore, the fresh air mixed with fuel in the combustion chamber is self-ignited at an ignition timing preferable for operation of the internal combustion engine. See, Japanese Patent Laid-Open Application No. Heisei 7-71279.
When ignition timing suitable for operation of an internal combustion engine is controlled positively so that active thermal atmosphere combustion occurs, the combustion is hereinafter referred to as AR combustion.
A spark ignition type 2-cycle internal combustion engine which allows AR combustion is illustrated in FIG. 11. In a low load operating region wherein the throttle valve opening .theta.th is restricted, thermal energy included in the combustion gas in a preceding cycle is utilized sufficiently to activate fresh air mixed with fuel in the combustion chamber. The internal combustion engine can operate in a combustion condition near to complete combustion. Therefore, the spark ignition type 2-cycle internal combustion engine exhibits a higher output power than that in an ordinary combustion condition which involves irregular combustion. As a result, where the internal combustion engine is carried on a vehicle or the like, it is disadvantageous. Even if the throttle valve opening .theta.th is restricted, while the internal combustion engine is operating in a high speed, high load operation condition, the effect of the so-called engine brake by a sudden drop of the output power of the internal combustion engine cannot be anticipated sufficiently. The above-mentioned high speed, high load operation condition may occur from attempting to stop the vehicle quickly.
Further, when the internal combustion engine rotates in a reverse direction upon starting or when a kill switch is disconnected to stop the internal combustion engine suddenly, if the internal combustion engine is in an AR combustion state, then it is difficult to quickly stop the internal combustion engine.